It has become standard practice in the trailer and trailer hitch industry to employ trailer hitch coupler designs of more or less standardized design, such that the trailer vehicle hitch coupler is readily attachable to trailer hitch balls mounted on a wide variety of towing vehicles. While this standardization has the advantage of allowing easy interchangeability of tow vehicles, it also has the corollary disadvantage of facilitating theft of unattached and unattended trailer vehicles. The advances of recent years in trailer hitch coupler designs which have made the coupling of trailer vehicles to tow vehicles a simple process have also made it possible for an unauthorized person to remove an unprotected trailer vehicle in a very short time.
A number of attempts have been made to provide devices intended to prevent trailer vehicles theft by locking the hitch coupler against attachment of a tow vehicle. Some of these attempts, as exemplified by U.S. Pat. Nos. 2,571,349 and 3,884,055, have provided relatively simple devices with externally disposed and unprotected locking means. Such devices, while easy to use, have the disadvantages of relatively loose fit and of failure to protect the locking means against tampering or removal with bolt cutters. Other approaches, as exemplified by U.S. Pat. Nos. 4,141,569 and 4,538,827, afford more locking means protection but suffer the disadvantages of reduced adaptability to different coupler designs and of higher cost associated with their more complex nature.